Interpretive Summary: Because visceral organs alter substrate supply to peripheral tissues and account for a major portion (50-70%) of whole body energy use, understanding factors that affect visceral tissue metabolism and growth is of great importance to nutritionists. Previous research by our group and others has demonstrated that visceral organ mass is responsive to level of alimentation and dietary chemical composition. This research extends these findings by demonstrating that increasing ruminal energy supply results in a disproportionate increase in digestive tract mass, due solely to increased rumen mass. In contrast, increasing small intestinal energy supply does not illicit changes in digestive tract organ mass. However, small intestinal energy supply, especially in the form of glucose, does result in greater adipose accretion compared to energy supplied ruminally. Under the physiological parameters of this experiment, omental as opposed to either subcutaneous or intramuscular adipose appears to be the preferred site for storage of excess glucose. Accordingly, current prediction models should be reparameterized to account for metabolic heat losses associated with maintenance of increased rumen tissue and differences in composition of accreted tissue observed with alterations in site of carbohydrate digestion.

Technical Abstract:
Forty crossbred beef steers with ruminal and abomasal infusion catheters were used to test two hypotheses: 1) visceral mass is responsive to energy input with extent of the response dependent upon site of carbohydrate (CHO) assimilation and 2) rate and site of adipose accretion is dependent upon site of CHO assimilation and CHO complexity. Treatments included a pelleted forage based basal diet fed at 161 (LI) or 214 (HI) Kcal ME/kg BW.75/d, LI plus ruminal (R-SH) or abomasal (A-SH) infusion of a partial starch hydrolysate , and LI plus abomasal infusion of glucose (A-G). Basal diet was fed in 12 equal portions daily at 2 h intervals. Starch and glucose were infused over a 22 h period daily at rates of 12.6 and 14.4 g/kg BW.75/d, respectively, to achieve isoenergetic infusions. After 35 d of infusion, steers were slaughtered and visceral organ and adipose (mesenteric and omental) mass were determined and subcutaneous adipose thickness over the 5th and 12th rib and longissimus intramuscular fat concentration were determined. Total intake energy (IE) was greater (P = 0.0001) for HI vs LI. Dietary IE was similar between LI and CHO infusion treatments, however total IE increased (P < 0.001) with CHO infusion. Total IE was similar across infusion treatments, however dietary IE tended to be lower (P = 0.10) for A-G vs A-SH. Greater dietary ME intake and CHO infusion tended (P < 0.09) to increase final BW and carcass weight. As a percentage of empty body weight (EBW), total stomach-complex, rumen, omasum, liver, pancreas, and kidney weights were greater (P < 0.05) for HI vs LI. Stomach complex, rumen, pancreas and kidney weights as a percentage of EBW were greater (P < 0.05) for R-SH compared with A-SH. Small and large intestinal mass as a percentage of EBW were largely unaffected by treatment. However, A-SH compared with A-G increased (P < 0.02) ileal total and epithelial weights from 10-cm sections. Increases in rumen mass were associated with either no change or an increase rumen total and epithelial DNA concentrations, thus rumen growth occurred via cellular hyperplasia. Greater dietary ME tended (P = 0.06) to increase subcutaneous fat thickness at the 5th rib but did not effect alimentary adipose accretion on an EBW basis. Omental and total alimentary adipose weights were increased (P <0.04) by A-G compared with A-SH. Although SH infusion did not significantly alter adiposity, there was a consistent numerical pattern in both total alimentary and subcutaneous fat depots with CHO infusion (A-G > A-SH > R-SH). Our findings demonstrate that increasing ruminal CHO supply results in a disproportionate increase in rumen mass, whereas increasing small intestinal CHO supply does not alter gut mass. However, small intestinal energy supply, in the form of glucose, results in greater adipose accretion, particularly the omental depot.